The present invention relates to external corrosion of insulated flowlines or pipelines caused by water saturation of the insulation and more particularly to the use of electrical resistance measurements to detect the presence of water in the thermal insulation at road crossings and other locations where the pipeline is inaccessible because it is protected by an external metal casing.
In areas with cold climates, it is often necessary to provide thermal insulation around oil and gas flowlines, or pipelines, to either prevent the oil products from becoming too viscous by chilling or to protect permafrost from thawing. The thermal insulation is normally held in place and mechanically protected by a thin metal sheathing. If the thermal insulation becomes water saturated, corrosion of the external flowline surface will result. Where the insulated flowline is physically accessible, infrared and/or neutron backscatter inspection techniques are commonly used to detect such water saturation without having to disassemble the insulation jacket.
In certain areas, such as road crossings, the insulated flowline must be buried. In such areas the flowline is mechanically protected by an additional heavy steel casing. One or more of such insulated pipelines are typically positioned within such casing by insulating support members which may be called stand-offs or spiders. In these areas, the flowline is not accessible and the usual water detection techniques cannot be used. Since the replacement costs of such a road crossing is high, on the order of $250,000, and requires more down time than an exposed flowline section, it is very important to detect the presence of water in the flowline insulation so that corrective steps can be planned before corrosion becomes dangerous.